1. Field of the Invention
The present invention relates generally to an actively controlled suspension system for an automotive vehicle, which controls suspension characteristics depending upon vehicle driving conditions in order to suppress attitude change of a vehicular body. More specifically, the invention relates to a hydraulic circuit construction with a fail-safe system in the actively controlled suspension system, which fail-safe system performs fail-safe operation in response to failure of components of the suspension control system, particularly electric circuitry.
2. Description of the Background Art
U.S. Pat. No. 4,702,490, issued on Oct. 27, 1987, which has been assigned to the common owner to the present invention, discloses a typical construction of an actively controlled suspension system, in which a hydraulic cylinder defining a working chamber is disposed between a vehicular body and a suspension member rotatably supporting a vehicular wheel. The working chamber of the hydraulic cylinder is communicated with a hydraulic circuit including a pressurized working fluid source. A pressure control valve, such as a proportioning valve assembly, is disposed in the hydraulic circuit, which is connected to an electric or electronic control circuit which controls the valve position. The pressure control valve is controls the valve position by a suspension control signal produced in the control circuit for adjusting pressure in the working chamber and thereby controlling suspension characteristics.
On the other hand, European Patent No. 0 283 004, 0 285 153 and 0 284 053 disclose technologies for controlling the suspension systems constructed as set forth above, depending upon the vehicle driving condition for suppressing rolling and/or pitching of the vehicular body.
As will be appreciated, for the automotive suspension systems, very high security has been required to assure safety in driving. Therefore, for the actively controlled suspension systems set forth above, it is important to assure safety at any vehicular driving condition. Furthermore, security is required for such electrically or electronically controlled suspension systems even when one of the components of the suspension control system fails.